Fuel tanks typically have a fill tube attached to the tank to facilitate filling of the tank with liquid fuel. In transportation vehicles, and in particular watercraft, these fill tubes must be sealed off with a cap to keep contaminants out of the tank and to prevent the escape of liquid fuel and vapors. To maintain air pressure in the tank at ambient air pressure, a vent line that opens to the atmosphere is installed in the top of the tank. This permits fuel to be added to or removed from the tank without meeting resistance from positive or negative air pressure. This also prevents the dangerous build up of air pressure due to expansion of fuel in the tank.
In a typical watercraft installation, the fuel tank vent line opens to the exterior of the craft, generally over the water. This type of venting has the serious drawback of permitting liquid fuel entrapped in the vent line to be vented overboard into the water. Entrapment of liquid fuel in the vent line frequently occurs during refueling when fuel in the tank foams and splashes into the vent line. This also occurs when the boat is rocking at the dock or is underway with near full tanks. The presence of fuel outside the fuel tank creates a risk of extensive damage to property and serious injury to life should the fuel ignite. Furthermore, the presence of liquid fuel in the water is known to result in environmental damage and the desctruction of marine life.